The advent of arcless current interruption by the use of solid state circuit components in combination with separable circuit interrupter contacts has substantially lessened the deterioration upon the contacts. This results in a smaller contact which in turn allows the contacts to open earlier in the current waveform at a lower current level. The reduction in circuit current during the interruption process synergistically allows less expensive solid state circuit components to be used within the solid state circuit interrupter. One such solid state circuit interrupter is described within U.S. patent application Ser. No. 610,947 filed May 16, 1984 entitled "Solid State Current Limiting Interrupter" in the name of E. K. Howell. This application is incorporated herein for purposes of reference.
The actual metal-to-metal interface through which current flows between a pair of electrical contacts is created by the force applied to hold the contacts together. The area (A.sub.c) of this conducting interface is determined by the applied closing force (Fc) and the hardness (H) of the contact metals as defined by the expression: EQU A.sub.c .alpha.Fc/H
Assuming the area A.sub.c to be circular, the corresponding radius: EQU a.sub.c =(A.sub.c /.pi.).sup.1/2= (Fc/H.pi.).sup.1/2.
The constriction of current through the area (A.sub.c) of radius (a.sub.c) results in an effective constriction resistance (R.sub.c). For homogenous material having a resistivity .rho.: EQU R.sub.c .alpha..rho./a.sub.c =.rho.(.pi.H/Fc).sup.1/2.
Therefore, in order to provide a low constriction resistance (R.sub.c), it is desirable to have a low resistivity material of low hardness and to apply a high closing force. Since the ratio of material hardness to closing force determines the constriction resistance, it follows that a material of reduced hardness allows use of a reduced closing force for any given resistance.
The lowest degree of hardness and the lowest force is obtained with a liquid metal, such as mercury, which has previously been used as a contact material. Mercury, which has a very high resistivity, presents additional problems when used as a contact material, such as maintaining a clean surface and confining the liquid metal. It is also difficult to deionize the mercury vapor arc which forms at high currents and voltages. Use of mercury-wetted, solid-metallic contact materials achieved low resistance by confining the high-resistivity mercury to a thin film between the lower-resistivity solid metallic contacts. This did not eliminate the surface contamination and arcing which vaporized and removed the mercury film.
An acceptable contact resistance should permit acceptable levels of current to flow without excessive voltage drop or heat generation. With commonly used contact materials such as silver, the resistance is primarily the constriction resistance, described earlier, such that most of the heat will be produced in the constriction area, raising the temperature of the contacts. Excessive temperature results in the rapid chemical reaction of the contact material with the surrounding atmosphere, and could melt the contact material and cause contact welding. It has since been observed that by increasing the current slowly through silver contacts, the temperature of the constriction area will reach approximately 180.degree. C., which is at the softening point of silver, reducing the hardness (H) of the contacts whereupon the closing force produces an increased area of conduction, thereby reducing the constriction resistance, which then remains reduced as the current is decreased. This yielding action produced by the reduced hardness of the contacts often results in a slight sticking or low-strength welding of the contacts upon cooling. If the current is raised rapidly, such that the closing force applied to the inertial mass of the moving contact cannot move the contact fast enough to increase the conduction area, surface melting, boiling or vaporization may occur resulting in damage and welding.
The speed of contact opening is determined by application of an opening force (Fo) which exceeds the closing force (Fc) to produce an accelerating force (Fa=Fo-Fc) acting upon the inertial mass of the moving contact. For high speed operation, it is desirable to reduce the mass and to reduce the closing force (Fc) as much as possible. The reduction in, and especially the elimination of arcing during contact separation has resulted in a significant reduction in the mass of the moving contact.
The purpose of this invention, therefore, is to reduce the closing force required for a given contact resistance to allow the contacts to be opened at a much faster rate.